1. Field of Endeavor
The present invention relates to a method for insulating a conductive bar.
In particular the present invention refers to conductive bars made of interwoven conductive strands (Roebel bars) having an insulating mica tape wrapped around them for use in high voltage large electric machines such as large generators.
2. Brief Description of the Related Art
Stator bars of large electric generators have a conductive bar (green bar) made of a plurality of interwoven conductors each provided with an insulating layer.
The conductive bar or green bar is provided with main insulation that is wrapped around it.
The main insulation is manufactured by wrapping one or typically more than one layer of a mica tape around the conductive bar.
The mica tape is thus impregnated with a resin that is then cured, in order to create a compact and stiff insulation around the conductive bar.
Usually, in order to impregnate the mica tape, either the Resin-Rich process or the VPI process are used; in the following, these processes are briefly described.
Resin-Rich Process
According to the Resin-Rich process, a mica tape having a backing layer and a mica layer pre-impregnated with a B stage impregnating resin in a large amount is used.
In particular, the mica tape is first wrapped around the conductive bar and then it is pressed, to expel the excess of B stage impregnating resin contained within the mica tape therefrom.
Thus the B stage resin is cured, in order to transform the B stage resin into A stage resin that has the mechanical and electrical properties required.
The Resin-Rich process proved to have these drawbacks: pre-impregnated mica tapes for Resin Rich impregnation are very expensive, and thus the conductive bars whose mica tape is impregnated this way are also consequently expensive; separate containers for warming up the pre-impregnated mica tape and curing the resin are required; and void filling is not optimal, since some voids are closed before impregnation by the adhesive tape.
VPI Process
In the VPI process (or its equivalent, TVPI; in the following reference to VPI will be made) a mica tape having only a backing layer and a mica layer is used (with a minimum amount of resin for bounding the mica and the backing layer together).
During manufacturing, the mica tape is wrapped around the conductive bar, creating one or typically more than one mica tape layer.
Afterwards the conductive bar with the dry mica tape wrapped around it is introduced into a tank wherein vacuum is applied in order to extract all gases (such as air) from the mica tape; then impregnating resin is introduced into the tank under pressure to impregnate the mica tape.
The VPI process also has a number of drawbacks; in particular: the VPI process requires a pressure resistant container that is very expensive; impregnation depends on the permeability of the mica tape; usually very thick mica tape layers are difficult to impregnate; and in case the mica tape contains filler, impregnation is further hindered and in addition the filler may be removed during impregnation.